Blastocystis hominis, a protozoan parasite of the human gastrointestinal tract and an organism implicated in the causation of human diarrhea, is being studied in terms of its morphology, physiology and drug sensitivity. This sporozoan may contain hundreds of mitochondria, and their enzyme activity, if any, in the subject of the study. B. hominis is sensitive to metronidazole, enterovioform, and other oxyquinoline derivatives. Drug testing is now being expanded to cover other appropriate drugs.